In making and using synthetic polymers, it is necessary to know and control the viscosity of the molten polymer stream within desired limits to maintain process continuity and to help control product quality and uniformity. On-line viscometers are known which divert a portion of a melted polymer stream through an instrument which measures, at a given temperature, the pressure drop across a restriction in the flow path under a particular initial pressure or at a particular flow rate. Using these values, the viscosity of the polymer may be calculated.
While viscosity measurements at a single, low shear rate are sufficient to control polymer processes where the quality of the polymer is consistent, these measurements are of questionable value when polymerization results in excessive branching of the polymer chains or in changes in molecular weight distribution. At low shear rates such branching may not have much impact on viscosity, but at high shear rates it does have a significant impact on viscosity. In polymer processes where final product uniformity is determined by high shear processing, such as fiber forming, viscosity measurements at multiple shear rates and interpretations of results becomes important in order to determine whether the branching level and the molecular weight distribution are within established desired limits. Heretofore the branching level and the molecular weight distribution, if measured at all, have been measured off-line, leading to costly delays and inefficiencies in the event such off-line testing indicated the need for correction.